A gamma ray camera of the positron type is being constructed. In such a camera two gamma ray detectors are placed on opposite sides of the radioactive source which emits positrons. The two annihilation gamma rays from the positrons are stopped by scintillators and two flashes of light are emitted. By locating the light flashes in the scintillators it is possible to deduce the location of the radioactive source. In the camera under construction the light flashes are located by placing an array of optical fibers against a face of each scintillator. The scintillator light can enter only the nearest fibers and so only a few fibers will transmit light to their other end. At the other ends of the fibers, photomultipliers convert the light flashes to electrical pulses. These electrical pulses are then used to determine the location of the light flashes in the two scintillators. The advantage of this type of camera is that optical fibers of l mm diameter (or less) can be used. This means that the light flashes can be localized to a region of a few millimeters. Furthermore, collimators are not required as that the sensitivity of the camera is not reduced because of the improved spatial resolution.